OTS (September 2008) -- In their FSI Knowledge Services™ Seminar presentation, Nirit Sofer and Lev Sherbelis of Numonyx confirmed significant yield gains in flash memory production after optimizing the SC1 cleaning process to increase particle removal efficiency (PRE), noting that the detailed optimization was enabled by the MAGELLAN® System’s flexible but precise recipe control of multiple process parameters (see Figure 1).
Figure 1: Optimized MAGELLAN® system process compared to wet bench process of record
Yields in flash memory production are much more sensitive to contamination by small particles than equivalent generation logic circuits, primarily because of differences in the geometry of the gate structures. Where the spacing between the gate contact and the source and drain contacts might be 130 nm in a logic circuit, it could be as little as 30 nm in an equivalent flash memory circuit. The smaller spaces can be bridged more easily by smaller particles, typically leading to shorting defects and device failure. Flash devices are also more sensitive to defects due to their higher operating voltage of 15 V, compared to 2 V for logic circuits. Because of these increased sensitivities, wet cleaning stations for flash manufacturing must be monitored for particles larger than 55nm, whereas the wet cleaning stations for equivalent generation logic manufacturing only need to be monitored for particles larger than 120 nm.
Sofer and Sherbelis optimized the flash device wet cleaning process on the FSI MAGELLAN® System for PRE relative to a number of operational variables, using the current wet bench process of record as a basis of comparison. The variables, all of which were under automated (recipe) control included:
Temperature – Final cleaning with SC1 depends on etching to remove particles from the surface, however, the device can be damaged if too much material is removed from the wafer surface. Both PRE and etch rate increase with temperature. Careful evaluation revealed an optimal temperature, which provided significant gains in PRE, while maintaining acceptable levels of material loss.
Megasonics – The megasonics evaluation compared varying power levels to the process of record (without megasonics). FSI’s unique MegaLens™ Technology allowed them to find an optimal process with high PRE and no damage to delicate structures at 1200 Watts.
Graded Recipe – The MAGELLAN® System’s precise control of chemical flow rates and concentrations was critical in developing a graded SC1 clean recipe for the hydrophobic post HF surface. In the graded step hydrogen peroxide is injected first to create a thin, protective layer of oxide before ammonium hydroxide is added to the mixture.
Wafer Transfer Speed – During and after HF treatment the hydrophobic wafers are particularly vulnerable to particle contamination. The MAGELLAN® System’s recipe flexibility allowed Sofer and Sherbelis to optimize the tank elevator and transfer robot speed when moving wafers out of the HF tank. They found that slowing the transfer in and out of the HF tank decreases defects.
Degasification – Gas dissolved in the cleaning solutions can increase pattern damage during megasonic treatment. Optimizing the recipe to eliminate multiple, and often unexpected, sources of dissolved gas significantly decreased megasonic induced defectivity.
Surface Tension Gradient Drying – FSI’s proprietary STG® drying technology eliminates water marks and similar contamination associated with the drying process. STG drying sets up an IPA concentration gradient in the extraction meniscus and the resulting surface tension gradient works with gravity to pull liquid more effectively from the wafer surface. STG drying in the MAGELLAN® System dramatically reduced defect counts when compared to the process of record.
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